1. Field of the Invention
The present invention is directed to a ventilator of the type having an inspiration section for supplying a respiratory gas to a patient's respiratory system during an inspiration phase, an expiration section for conveying respiratory gas from the respiratory system during an expiration phase, a control unit for controlling the inspiration section and the expiration section and a detector means for detecting spontaneous inhalation by the patient, connected to the control unit.
2. Description of the Prior Art
A ventilator of the above type is commercially available, known as the Servo Ventilator 300, described in an Operating Manual, Siemens-Elema AB, AG 0593 3.5, May 1993. The Servo Ventilator 300 can be set to operate in a number of different ventilation modes, such as pressure control, volume control, volume support, pressure regulated volume control, etc. One feature, selectable in all ventilation modes, is the trigger function, i.e., automatically triggering an inspiration phase dependent on the patient's breathing efforts. The ventilator detects attempts of the patient to breathe spontaneously and delivers, upon such detection, a controlled or supported inspiration (depending on the set ventilation mode). When the patient triggers in a controlled ventilation mode, the patient will be provided with a controlled breath according to the set parameters for the control mode. If such triggering occurs in a support ventilation mode, the patient may exercise some control of the respiration cycle. Some control and support modes also include a control feature for insuring that the patient receives a sufficient tidal volume or minute volume of respiratory gas.
It is also possible that the patient may try to inhale spontaneously during a controlled inspiration phase. If the pressure level is regulated in the control mode, the pressure level will be maintained by the ventilator and the inspiration phase will be completed at the same pressure level. If the tidal volume supplied to the patient is controlled, the ventilator will determine whether the patient can complete the breath, i.e., whether the patient by means of the spontaneous inhalation can reach (inhale) the set tidal volume. If the patient is incapable of completing the breath, the volume control will extend throughout the entire inspiration phase. If the patient is capable of completing the breath, the inspiration phase will be terminated and a normal exhalation phase commences.
In order to increase comfort for the patient and to accelerate any weaning of the patient from the ventilator, the supply of respiratory gas must be made in a way which guarantees a sufficient supply of respiratory gas to the patient but at the same time allows a respiration pattern which is as normal as possible for the patient. The patient will thereby be stimulated to increase spontaneous respiration and reduce dependency of the ventilator for maintaining a sufficient respiration.